1. Field of the Invention
The present invention generally relates to semiconductor integrated circuits and other connection arrangements which include fuses for programming thereof and, more particularly, to high density circuit structures utilizing low dielectric constant materials.
2. Description of the Prior Art
It has long been recognized that increased density of circuit formation in integrated circuits and interconnect structures such as printed circuit boards yields potential increases in performance and functionality as well as manufacturing economy. However, increased density is generally accompanied by increased proximity of current-carrying structures which can compromise performance by increasing capacitance and capacitive coupling of such structures.
Therefore, there has been substantial recent interest in insulators having particularly low dielectric constants for use between conductors in order to reduce capacitance and capacitive coupling. These materials often present additional problems of strength and/or thermal stability which may also compromise manufacturing yield and, hence, manufacturing economy, as well as some potential problems of long-term reliability.
For example, an organic polymer insulator known as xe2x80x9cSiLKxe2x80x9d (for silicon low-k; a misnomer since it contains carbon but no silicon), commercially available from Dow Chemical Co., exhibits a suitably low dielectric constant and is easily applied in liquid form by well-known spin-on techniques and cured at a temperature of about 400xc2x0 C. However, this material will pass through a glass transition at roughly 450xc2x0 C., causing changes in stress. This stress could cause damage in adjacent electrically active layers or cause delamination of the SiLK layer. If any of these processes degrade the structure or allow cracks to develop in barrier layers, the chip may fail during use.
The SiLK layers are also poor barriers to oxygen diffusion, and if oxygen reaches metal conductor layers such as copper metallization, corrosion can result; compromising electrical integrity or at least increasing resistance (and propagation time) of the affected conductors. Since SiLK is a relatively viscous film, cracks in the SiLK may or may not propagate, and such cracks are likely to terminate at the edge of a harder layer such as the nitride barriers. Degradation is more likely to occur because of a catastrophic event at high temperature rather than a crack propagation event.
It is also customary at the present time to provide fuses or antifuses in integrated circuit devices and other connection arrangements. Such devices may be used, for example, to alter the function of the overall circuit or portions thereof or to substitute redundant circuits for others having defects or reduced operating margins in order to enhance manufacturing yield. Localized heating of metal fuses using lasers at higher levels of material have typically been used in the past, but more recently, silicide materials such as cobalt silicide have been favored due to improved scaling capability, compatibility with standard processing and ability to maintain an oxygen and water diffusion barrier.
An exemplary silicide fuse is disclosed in U.S. Pat. No. 5,708,291 which is hereby fully incorporated by reference. Passing a large current per unit cross-sectional area causes a temperature rise in the silicide, allowing the silicide to degrade through electro-migration, silicide agglomeration or catastrophic failure. This causes the bulk resistance to rise sufficiently (e.g. about ten fold) for the state of the fuse to be readily distinguished.
For example, in integrated circuits at the present state of the art, a fuse can be fabricated of a silicide film conductor which is (or may be shaped to be, as disclosed in U.S. Pat. No. 5,882,998, also fully incorporated by reference) about 80 nm wide and 30 nm thick in the fuse region. Thus, a current on the order of milliamperes results in a current density on the order of 50 MegAmperes/cm2. Therefore, while the silicide has a relatively low initial bulk resistance, although somewhat higher than metal, substantial heating and large temperature rise will be caused in the course of programming of a fuse.
In general, an oxide and/or nitride dielectric will be employed immediately adjacent to the fuse material but usually in a relatively thin film. Thus, if a low-k dielectric is employed, it will be overlaid on that relatively thin film and thus be in very close proximity to the fuse element. Moreover, low-k dielectrics and SiLK in particular have very low thermal conductivity and can thus prevent or reduce heat transfer by which temperature rise can be limited. Therefore, low-k organic materials proximate to a fuse will be sensitive to degradation during fuse programming. Since a fuse element may be used to provide redundancy in an integrated circuit to enhance manufacturing yield, many thousands of fuses may be provided and programmed after testing. Since a 1% yield failure is very significant, a 0.1% fuse reliability failure would be intolerable since such a rate would correspond to one or more instances on every chip where redundant circuits could not be substituted, if needed.
One approach to thermally protecting an inorganic layer is disclosed in U.S. Pat. No. 5,389,814 to Srikrishnan which is assigned to the assignee of the present invention and fully incorporated by reference. In accordance therewith, the oxide insulator layer placed over the fuse element has a thickness of at least (C)xc3x97(fuse thickness)xc3x97(specific heat ratio of fuse to heat shield (e.g. oxide) material, where C is the ratio of the fuse melting/degradation temperature to the degradation temperature of the organic low k insulator material and generally has a value of roughly 5. This relationship generally leads to an insulating heat shield thickness of greater than 100 nm which may be suitable for some printed circuit board applications, as disclosed therein. However, such a thickness is larger than desired for semiconductor processing when manufacturing tolerances are considered.
It should also be understood that putting fuses at a higher level in an integrated circuit causes increased likelihood that oxygen will enter from the chip surface to reach the fuse. Additionally, the processing sequence generally makes placement of fuses at lower levels preferable. For example, placement of fuses at higher levels requires additional masking layers. Therefore, it is desirable to put fuses at the lowest level possible. However, such placement is not generally consistent with thicker heat shields.
Evaluation of fuses created with such a silicide fuse show various sorts of degradation with occasional cracking of the hard dielectric adjacent to the fuse structure. Such a crack must not be allowed to propagate to higher levels, particularly copper levels where electrical integrity is important. However, no structure suitable for placement adjacent a fuse element that provides a location for termination of cracks has heretofore been known.
It is therefore an object of the present invention to provide a structure suitable for use at any level in an integrated circuit having high integration density and including low-k dielectric materials which provides a heat shield and crack stop protection and limits temperature rise during fuse programming by efficient heat sinking.
It is another object of the present invention to provide a fuse structure that can be fabricated at very small size and including a silicide fuse element that can be fabricated near the substrate in, for example, an integrated circuit and thus exploit the heat conduction properties of the substrate to limit temperature rise and damage and to enhance device reliability.
It is a further object of the invention to provide a structurally robust integrated circuit and/or interconnect structure including fuse elements and having decreased signal propagation time and noise immunity by virtue of decreased capacitive coupling between conductors that may be very closely spaced and shielding as well as improved heat dissipation and heat control.
In order to accomplish these and other objects of the invention, an electronic device is provided including a heat producing element such as a fuse, a layer of dielectric material overlying the heat producing element, a layer of low-k dielectric material overlying the layer of dielectric material, and a layer of thermally conductive material interposed between the dielectric material and the low-k dielectric material to provide a heat shield for the low-k dielectric material.